Electromagnetic and thermal shield for electronic energy meter
BACKGROUND OF THE INVENTION 
     The present invention relates to an electromagnetic and thermal shield for an energy meter having a transparent glass or polycarbonate housing, particularly where the energy meter is of the electronic type. 
     Energy meters are installed and utilized in millions of different locations, many of which are exposed to very high temperatures, including direct sunlight, and also exposed to electromagnetic interference which could adversely affect the operation of the electronic energy meter. Direct sunlight shining through the transparent housing results in a greenhouse effect in which the interior of the energy meter tends to reach extremely high temperatures, much higher than the ambient temperature outside the meter. 
     While it is conventional to have a transparent housing for an energy meter to permit the reading of the visual displays of the energy meter by the customer or meter reader, it is not desirable to have the component parts, including the electronics of the meter, visible from the outside, not only from a heat and electromagnetic radiation point of view, but also from an aesthetic point of view. 
     Various approaches to providing electromagnetic and thermal shields for energy meters have been proposed or utilized, including that disclosed, for example, by U.S. Pat. No. 4,795,975 in which a thermal and electromagnetic shield is fabricated of organic layers bonded to a foil layer through use of intermediate adhesives. The resultant pliable thermal and electromagnetic shield presents possible problems of grounding, retention of shape, and concern that the pliable shield might accidentally make electrical contact with voltages within the energy meter. It is desirable that the electromagnetic and thermal shield be rigid and provide a positive ground on contact. 
     In addition, the energy meter may be constructed in a layered configuration, that is, with a base assembly and a register or meter assembly spaced from the base assembly with an intermediate electronic layer. In such meters means must be provided to support these multiple layers relative to one another, and also to prevent dangerous shock from the voltages present within, and between, these layers when the cover is removed, for example, to actuate certain test or other functions built into the meter. 
     OBJECTS AND SUMMARY OF INVENTION 
     One object of the present invention is to provide an improved thermal and electromagnetic shield for an energy meter. 
     Another object of the present invention is to provide an electromagnetic and thermal shield for an electronic energy meter which is readily grounded, is non-transparent, and heat-reflective. 
     Still another object of the present invention is to provide a thermal and electromagnetic shield for an electronic energy meter which is readily assembled and disassembled and which provides rigidity and support to the multiple layers of components within the electric energy meter, and which also acts as a safety shield when the cover of the meter is removed. 
     In accordance with a preferred embodiment of the present invention, an electromagnetic and thermal shield is provided for an electronic energy meter including a base for connection in circuit with the power source and load to be metered, a register spaced from the base, intermediate electronics, and a transparent cover. The tapered shield extends from the register to a rim in the base and includes intermediate resilient teeth formed from the shield to grip contact portions on a transverse support member between the base and register. The teeth extend inwardly from the shield and include rounded teeth on the bottom with the opposed top teeth having a flatted portion with a tab extending parallel to the base, to position and secure the shield and to provide shielding and support for the components of the meter. The shield is formed of sheet steel with the grain parallel to the base and with a smooth outer seam perpendicular to the base provided by offset welded tabs which pass under the seam. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified drawing of a cross section of an electronic energy meter incorporating the invention. 
     FIG. 2 is an enlarged view of the electromagnetic and thermal shield of FIG. 1 and including an angular positioning guide. 
     FIG. 3 is an enlarged view showing the seam of the electromagnetic and thermal shield of FIGS. 1 and 2. 
     FIG. 4 is a top view of the electromagnetic and thermal shield of FIG. 3. 
     FIG. 5 is an enlarged view showing the fastener for securing the electromagnetic and thermal shield to an intermediate layer of the energy meter. 
     FIG. 6 is an enlarged side view of the fastener shown in FIG. 5. 
     FIG. 7 is a top view of the plate in the intermediate layer of the energy meter to which the electromagnetic and thermal shield is fastened. 
    
    
     Referring to FIG. 1, an electronic energy meter 2 is shown in simplified form and includes a base 4 having a plurality of meter circuit terminals such as 6 and 8 extending through the base 4 to connect the meter in circuit with a power source and a load which is to be metered. A register 10 including a digital readout (not shown) is positioned at the other end of the energy meter 2, with an intermediate electronic voltage assembly 14 including a printed circuit board 16 carrying electronic components shown generically as 18 and 20 and supported by transverse support member 22 which, in turn, is supported by spacers such as 26 and 28 interposed between the base 4 and the transverse support member 22. Support members 32 and 34 are interposed between the transverse support member 22 and the register 10. The base 4 also supports the current transformers or sensors shown generally as 36. A transparent cover or housing 38 surrounds the meter components, and in combination with the base 4 encloses the assembly. The transparent cover housing 38 is commonly made of glass or a transparent polycarbonate. 
     Details of the construction of an electronic energy meter 2 which incorporates the present invention are set forth in copending U.S. patent application, Ser. No. 411,934 filed by W. R. Germer, M. J. Ouellette, D. F. Bullock and A. F. Palmer, assigned to the same assignee as the present invention and hereby incorporated by reference. 
     As discussed above, the &#34;greenhouse&#34; effect of the transparent cover tends to build up extremely high temperatures within such an energy meter. In order to minimize such temperatures, and also to isolate the electronic components such as 18 and 20 and those associated with the register 10 from electromagnetic interference, an electromagnetic interference (EMI) and heat shield 44 is positioned around the energy meter components and within a transparent housing 38. 
     The EMI and thermal shield 44 is generally cylindrical in shape and extends between a shoulder or step 82 in base 4, and the top edge 48 of the molded bezel 50 associated with the register 10. The EMI and thermal shield 44 is shown in FIGS. 2-6. Referring first to FIG. 2, the EMI and thermal shield 44 is formed of sheet brushed finish stainless steel, 0.010 inches thick, punched and fabricated to the shape shown, with the ends curved around to meet one another to form a cylindrical sleeve. The axial edges or ends 54 and 56 are drawn around to meet, with the tabs 60 and 62 which extend out from axial end 54 being positioned under the axial end 56. The ends 54 and 56 are held in place by spot welds 64 and 68 for tab 60, and spot welds 72 and 74 for tab 62. Matching holes 79 and 81 in tabs 60 and 62, respectively, and in the top and bottom region adjacent axial edges 54 and 56, are used to position the axial ends during the spot welding process. The inside diameter of the bottom 78 of the EMI and thermal shield 44 is 5.916, inches while the inside diameter of the top 80 of the EMI and thermal shield is 5.500 inches providing a slight inward taper toward the top of the EMI and thermal shield. This facilitates assembly while insuring a snug fit. The bottom 78 of the EMI and thermal shield can be readily slid over the smaller diameter bezel 50 of the register 10, to fit snugly around step 82 in the base 4 (best shown in FIG. 1), and with the smaller diameter top edge 80 fitting snugly around the bezel 50 when the EMI and thermal shield is slid all the way down to the groove in the base. 
     A generally rectangular shaped cutout 84 is provided for optical visibility of an optical bar code reading arrangement which is described in more detail in the afore-referenced co-pending patent application No. 411,934. 
     FIG. 3 is a cross-sectional view taken along lines 3--3 in FIG. 2. Referring to FIG. 3, it is to be noted that the offset 150 brings the tab 60 behind the axial edge 54 such that the outside surface of the EMI and thermal shield 44 in the region of axial edges 54 and 56 is substantially continuous, without an outside step which might otherwise result if there were a simple overlap of the ends. The formation of the offset 150 is facilitated through use of small notches such as 152 and 154 (shown in FIG. 2) at the juncture of the edge 54 and the outwardly extending edges of the tab 60. This facilitates the bending of the tabs without deforming the remainder of the EMI and thermal shield 44. Holes 79 assist in positioning the axial edges 54 and 56 during the spot welding process. 
     The EMI and thermal shield 44 includes 3 fasteners 90, 92 and 94 equally spaced about the circumference of the EMI and thermal shield, that is, spaced 120 degrees apart, for positive fastening of the heat shield about, and to, the transverse member 22 shown in FIG. 1. The spacing of the fasteners 90, 92 and 94, are shown in FIG. 4. In order to enhance the strength and rigidity of the EMI and thermal shield 44, the fasteners 90, 92 and 94, and optical cutout 84, are positioned and spaced from the seam 98 where the inner edges of tabs 60 and 62 underlie axial edge 56. The angle 102 between seam 98 and the center of fastener 90 is 40 degrees, while the angle 106 between seam 98 and the center of fastener 94 is 80 degrees. The angle 104 between the seam 98 and the closest edge 105 of the optical cutout 84 is 15.5 degrees and the arc 108 spanning the optical cutout 84 is 36 degrees. The direction of the grain of the stainless steel forming EMI and thermal shield 44 extends parallel to the bottom edge 78, that is, parallel to the base 4 and the register 10 to further facilitate bending the steel into the desired configuration. 
     The configuration of the fasteners 90, 92 and 94 are shown in FIGS. 5, 6 and 7. Referring to FIGS. 5, 6, and 7, and first to FIG. 5, it is seen that the fastener is formed by punching out a pair of Y-shaped segments 112A and 112B to form tooth 116 and tooth 118 with root space 120 therebetween, and having flatted ends 124 and 126, respectively The rounded tooth 130 is positioned opposite the root space 120 and is surrounded on either side by root spaces 134 and 136, which are the legs of the Y-shaped segments 112A and 112B. Thus, the spaced flatted teeth 116 and 118 are opposed by the generally rounded tooth 130. 
     The teeth 116, 118 and 130 are bent inwardly from the EMI and thermal shield 44 to form gripping surfaces for gripping the transverse support member 22 as shown in FIG. 6. Referring to FIG. 6, which is an enlarged cross-sectional view taken along the lines 6--6 in FIG. 5, tooth 118 and tooth 130 are bent inwardly from the side of the EMI and thermal shield 44. Tooth 130 is bent inwardly from the EMI and thermal shield at an angle 139 of 20 degrees, while tooth 118 is bent inwardly at an angle 140 of 12 degrees. A generally flatted portion 142 of tooth 118 extends inwardly 0.040 inches substantially perpendicular to the EMI and thermal shield 44. The distance between the flatted portion 142 of tooth 118 and the end 146 of rounded tooth 130 is 0.042±0.005 inches which is less than the thickness of the transverse support member 22 which is 0.044 inches thick. 
     In assembling the EMI and thermal shield 44, the shield is slid down and past register 10 with the enlarged bottom 78 passing freely past the register When the fasteners 90, 92 and 94 approach transverse support member 22, their inwardly extending rounded teeth, such as tooth 130, contact the edge of the transverse support member 22 and are pressed outwardly toward the EMI and thermal shield 44. Further movement downward of the EMI and thermal shield 44 moves the transverse support member 22 into contact with the flatted portions, such as 142 of the teeth 116 and 118, which act as resilient stops, at which time the resiliency of rounded tooth 130 forces the rounded tooth to spring inward under the support member securing the support member between the rounded tooth 130 and the flatted portion 142 of tooth 116. This action is repeated by all three of fasteners 90, 92 and 94 to secure the EMI and thermal shield 44 to the transverse support member 22, and to ground the shield through the metallic and conductive transverse support member which is grounded At the same time, the bottom 78 of the EMI and thermal shield 44 is firmly supported in the step 82 of base 4, and the narrower top 80 of the EMI and thermal shield securely surrounds bezel 50. Thus, the EMI and thermal shield 44 provides support to, and in turn, is supported by the base 4, the transverse member 22 and the bezel 50. The cutouts 158 at the top 80 of the EMI and thermal shield 44 facilitate the fastening of the nameplate (not shown) to bezel 50. 
     In a preferred embodiment of the present invention, the angular positioning of the EMI and thermal shield 44 on the electronic energy meter 2 is facilitated by the guide 160 (see FIG. 2) which extends upward from step 82 (see FIG. 1) and which is molded as part of base 4. A rectangular cutout 162 which is approximately twice the width, and approximately the same height, as guide 160 in the bottom edge 78 of the EMI and thermal shield 44 passes around the guide to provide the proper angular positioning. The EMI and thermal shield 44 is positioned and rotated about guide 160 with the assembled position of the EMI and thermal shield relative to the guide shown in FIG. 2. FIG. 7, shows the aluminum transverse support member 22. In the assembled position, the teeth 116, 118, and 130 of each fastener 90, 92 and 94 grip the circumferential area of the transverse support member 22 in the contact area regions 170, 172 and 174, respectively. The circumference 176 of the transverse support member 22 is generally circular in shape with a number of cutouts or notches such as notches 182, 184 and 186 adjacent contact areas 170, 172 and 174, respectively. Rotation of the EMI and thermal shield 44 in a counterclockwise direction rotates the teeth of fasteners 90, 92 and 94 out of frictional gripping engagement with the top and bottom of the transverse support member 22 by moving the bottom teeth 130 into the notches 182, 184 and 186, respectively, after which the EMI and thermal shield may be slid upward from around the base 14, electronic voltage assembly 14 and bezel 50. 
     Since the width of the cutout 162 is approximately twice that of the guide 160, it provides a positive stop for rotation of the EMI and thermal shield, first on one side 190 for the assembled position shown in FIG. 2, and then on the other side 192 when the side 194 of the cutout 162 of the EMI and thermal shield 44 is rotated in the counterclockwise direction to disengage the fasteners 90, 92 and 94 in preparation for removing the EMI and thermal shield Assembly of the EMI and thermal shield 44 can best be accomplished by a reversal of the procedure described above That is, the EMI and thermal shield is slid down over the bezel 50 and electronic voltage assembly to base 14 with the side 194 of cutout 162 adjacent the side 192 of guide 160 such that the bottom teeth 130 of fasteners 90, 92 and 94 are in the notches 182, 184 and 186, respectively, and out of contact with the transverse support member 22. After the EMI and thermal shield 44 is seated in the step 82, it is rotated clockwise so that the teeth 116, 118 and 130 are slid into frictional gripping engagement with the bottom and top of the transverse support member. 
     The close fit between the circular segments such as 176 of the EMI and thermal shield 44 and the planar transverse support member 22 provides strength and rigidity to the EMI and thermal shield and helps prevent denting of the relatively thin EMI and thermal shield when the energy meter 2 is handled or serviced with the cover 38 removed. The steel EMI and thermal shield 44 thus provides electromagnetic shielding by grounding through transverse support member 22, provides support and strength to the electronic energy meter 2, is thermally reflective, aesthetically covers and obscures the interior of the energy meter even with a transparent cover 38, and in addition provides a grounded safety shield when the cover is removed by a serviceman preventing the fingers of the serviceman from contacting the voltages present at many places within the electronic energy meter. Thus, while the invention has been described through preferred embodiments, such embodiments are provided by way of example only. Numerous variations, changes and substitutions, including those discussed above, will occur to those skilled in the art without departing from the scope of the present invention and following claims.